The solid waste generated in hydrometallurgical processes, such as different kinds of iron deposits and leach residues, usually contain small amounts of soluble heavy metals, like zinc, cadmium, cobalt, nickel, arsenic and antimony. These kinds of residues require pre-treatment, in which they are stabilised before storage at a landfill site, so that the heavy metals do not dissolve from the waste. Known pre-treatment methods carried out either separately or together include for instance waste washing, neutralisation and precipitation of the metals as hydroxides, precipitation of the metals as sulphides, isolation of the waste site from the groundwater and binding the soluble compounds with for example, cement, phosphate or lime.
Sulphide precipitation is one effective method for binding heavy metals, but the additional costs incurred by the method as well as the large amount of water migrating to the landfill site may be considered a weakness. Due to the large quantity of water involved, multi-layered walls and a water collection system have to be constructed on the landfill site to prevent the water at the site from seeping into the groundwater.
The zinc production process is one typical process in which an iron-bearing waste is generated. The production process originating from zinc sulphide concentrate according to one approach comprises roasting of the concentrate, leaching of the calcine, i.e. zinc oxide that is obtained, where the zinc oxide is leached with a solution containing sulphuric acid to form a solution of zinc sulphate in what is called neutral leaching. The zinc sulphate solution is routed generally via solution purification to electrolytic recovery. The insoluble residue of neutral leaching consists of the zinc ferrite and sulphur formed in roasting, and the residue is treated in a strong acid leaching stage to leach the ferrite, so that the zinc bound to it is recovered. Iron is precipitated as jarosite, goethite or hematite, most commonly as jarosite. Often the residue is subjected to flotation to separate sulphur from the iron deposit. Zinc sulphide concentrate can also be routed for example to the strong acid leaching stage without roasting or the entire concentrate leaching can be performed without roasting and the waste residue that is generated contains both the iron and the sulphur of the concentrate.
The disposal of the iron residue generated in the leaching process of zinc concentrate and other equivalent metals should occur so that the final residue or reject is as poorly soluble as possible, whereby any small heavy metal residues that may have remained in it do not cause problems. Hematite is very poorly soluble, but its production generally requires autoclave conditions, which raise the costs of the process.
There have been attempts to solve the iron residue storage problem e.g. as presented in CA patent publication 1079496 and the publication by Ek, C. “Jarosite treatment and disposal by the ‘Jarochaux’ process,” Int. Symposium on Iron Control in Hydrometallurgy, Oct. 19-22, 1986, Toronto, Part. VII pages 719-729, which describe the Jarochaux process. According to this method, an iron residue, which may be jarosite or other possible iron compounds, is mixed with a calcium compound. The calcium compound may be for example quicklime, slaked lime or lime milk. As a result of the physicochemical reactions spherical lumps are formed, with a diameter of 1-20 cm. The sulphate in the iron residue reacts with the calcium and forms gypsum, which in turn forms a skeleton inside the jarosite lump and a shell around the lump. The method consists of the following stages: the first stage is filtration, followed by elutriation to a solids content of about 50 g/l, after this thickening and filtration of the thickener underflow (solids content approx. 200 g/l), air drying of the residue on a filter, after which the moisture content is about 35%. From the filter the residue is routed by belt conveyor to a screw mixer, into which dust-like lime is also fed. When the iron residue is mainly jarosite, the amount of lime (CaO) to be added is 6-16% of the quantity of dry solids of the waste residue. When the waste residue is goethite, the amount of lime required is smaller. According to the examples in the patent publication, the mixing reactor for the residue and lime is launder-shaped and equipped with two blade mixers rotating opposite each other.
According to the method described in IT patent publication 1290886, waste containing heavy metals is stabilised by adding calcium hydroxide, orthophosphoric acid or its salts into the waste as an aqueous solution, and if necessary water, in order to obtain a paste of uniform consistency. The drawback of this method is that the waste has to be dried before storage at the landfill site.
Lime neutralisation is suitable for almost all kinds of wastes and even old landfill sites can be treated by the addition of lime. However, the method has the disadvantage that the waste generated is not of uniform quality. As a result of non-uniform neutralisation, some of the material remains un-neutralised and in some of the material the pH can rise so high that it causes the decomposition of the jarosite.
Yet another method intended for the disposal of iron residue, especially jarosite, is the Jarofix process, which is described for example in the article by Seyer, S. et al: “Jarofix: Addressing Iron Disposal in the Zinc Industry”, JOM, December 2001, pages 32-35. The initial part of the method is similar to that of the Jarochaux process described above, i.e. the jarosite residue is elutriated, thickened and lime is mixed into the residue, but after this cement is further added to the residue to bind the residue. Cement enables the long-term physical and chemical stabilisation of iron residue. Of course the use of cement as a binding agent stabilises jarosite well, but it also causes extra costs for the process.
FI patent publication 84787 has disclosed an mixing reactor and a mixer located in it, and the apparatus is intended for mixing two liquids into each other or a liquid and solid and simultaneously separating from the liquid either the other liquid or the solid. The apparatus is made up of a three-part reactor, the upper section of which is cylindrical, the section below it conical and the lowest a tubular collection part. Baffles are positioned on the edges of the reactor. The mixer consists of two tubular coils surrounding the shaft and a protective cone fixed in the lower section of the mixer, which is intended to prevent the flows from entering the reaction zone and sucking drops of liquid upwards. The diameter of the mixer is 0.5-0.75× the diameter of the reactor, which means that in practice the agitated zone is only half the volume of the reactor. The mixer also extends into the conical section of the reactor and the distance of the tubular coils from the mixer shaft decreases correspondingly so that the ratio of the mixer diameter to the reactor diameter remains at the previous level. The reactor and mixer are intended for mixing either two liquids or a liquid and a solid and the description of the equipment reveals that the solids content of any slurry that may be generated is not very high. The mixing in the lower section of the mixer is weaker, so the phases separate after the reactions that have occurred during mixing. In the lower section of the reactor the aim is to prevent solids from migrating to the upper section of the reactor.